After two times of refined salt aqueous solution of continuous into the anode chamber, and sodium ion in the electric field under the action of moving through a cation exchange membrane to the cathode chamber, into the cathode liquid sodium ions with cathodic electrolysis of water and produce hydrogen and oxygen ions generated on sodium hydroxide, and hydrogen at the cathode. The chloride ions in salt water solution are limited by the membrane and can't enter the cathode chamber and be oxidized to chlorine gas at the anode. After partial sodium chloride electrolysis, the remaining brine will flow out of the electrolytic cell and return to the anode chamber after the removal of dissolved chlorine, resaturation of solid salt and purification, thus forming a brine loop similar to mercury method. Sodium hydroxide solution leaving the cathode chamber is part of the product, part of which is added with pure water and returned to the cathode chamber. The circulation of lye helps to control the amount of water added accurately, and also takes away the heat generated in the electrolytic cell.